A rocket motor nozzle includes a quantity of nozzle material arranged around a nozzle passage. In flight, a rocket motor adjacent to the nozzle creates combustion products which are forced under pressure through the nozzle passage out to the rocket's ambient environment. As the combustion products pass through the nozzle passage, they exert erosive forces on the nozzle material. Conventional nozzles attempt to substantially eliminate erosion of the nozzle material by the combustion products.
Control over nozzle erosion is sought because erosion may affect the thrust created by the rocket motor and nozzle. Erosion control is most important at the "throat," which is the portion of the nozzle passage having the smallest cross-sectional area. In a conventional nozzle, the location of the throat along the longitudinal axis of the rocket does not change substantially during flight. However, the size of the throat may increase as erosion wears away part of the nozzle passage.
In general, increasing the size of the throat tends to decrease the pressure within the combustion chamber, and hence decreases the impetus on combustion products to exit the chamber. This decrease in turn reduces the rocket's thrust, because thrust =mass change.times.velocity. That is, the thrust created equals the product formed by multiplying the total mass of the combustion products exiting the rocket per unit of time by the velocity of the exiting products. In short, increased erosion tends to increase the diameter of the nozzle's throat and hence tends to decrease the thrust produced.
The erosive forces act directly on the throat because the combustion products usually reach critical velocity (Mach one) within the throat. However, the extent to which combustion products erode the nozzle material depends on several factors. Some of the principal factors affecting nozzle erosion are the pressure within the nozzle passage, the temperature of the combustion products, the chemical composition of the nozzle material, the chemical composition of the combustion products, and the duration of the "burn" which produces the combustion products.
To maintain a substantially constant nozzle throat diameter, many conventional nozzles utilize nozzle materials chosen for their ability to resist erosion. For instance, some nozzles include an erosion-resistant liner which surrounds at least a portion of the nozzle passage. The nozzle passage liner is typically formed of "non-eroding" materials such as carbon--carbon, tungsten, or molybdenum. One such conventional nozzle is used in tactical rockets, including tactical non-line-of-sight ("NLOS") rockets which have sustained burn times in excess of 100 seconds.
The conventional NLOS nozzle includes a metallic structural shell disposed around concentric layers of nozzle material. The outer layer of nozzle material includes an insulator, such as an alumina-silica ceramic. The inner layer of nozzle material includes a substantially non-eroding nozzle passage liner, such as a molybdenum liner. The nozzle is attachable to a rocket motor.
The nozzle includes an entry, a nozzle passage, and an exit orifice. During flight, combustion products from a rocket motor's combustion chamber enter the entry, pass through the passage, and exit through the orifice into the ambient environment. As noted, the throat is defined as the smallest part of the passage. Erosion of the liner about the throat is limited by the erosion-resistant nature of the material used in forming the liner.
Although nozzle passage liners formed of carbon-carbon, tungsten, molybdenum, or similar materials minimize erosion, they are also relatively expensive. For example, in a typical NLOS rocket, a molybdenum liner may account for one-fourth or more of the total cost of the rocket motor. Moreover, few such rocket motors are typically recovered, so the high cost of such non-eroding liners cannot be spread over many uses of the nozzle.
Thus, it would be an advancement in the art to provide a rocket motor nozzle which allows a rocket motor to control nozzle passage erosion without requiring the use of an expensive nozzle passage liner.
It would also be an advancement in the art to provide such a nozzle which is suitable for use in tactical rockets, including tactical NLOS rockets which have sustained burn times in excess of 100 seconds.
Such a rocket motor nozzle is disclosed and claimed herein.